The application generally relates to a capacity control system for a centrifugal compressor having one or more stages. The application relates more specifically to a capacity control system for a compressor having a Variable Geometry Diffuser (VGD). In a refrigeration system capacity refers to cooling ability, while in other gas compression systems, capacity refers to volumetric flow.
Previously, in centrifugal compressors for use in cooling systems, such as liquid chillers, refrigeration or heating, ventilation, air conditioning and refrigeration (HVAC & R), and gas compression, pre-rotation vanes (PRV) or inlet guide vanes were required for controlling the cooling capacity of the cooling system. At the inlet to the compressor from an evaporator, one or more PRV control the flow of refrigerant to the compressor. An actuator is used to open the PRV to increase the amount of refrigerant to the compressor and thereby increase the cooling capacity of the system. Similarly, the actuator is used to close the PRV to decrease the amount of refrigerant to the compressor and thereby decrease the cooling capacity of the system.
VGDs have been used for controlling rotating stall in the diffuser region of the centrifugal compressor. One embodiment of a VGD is described in U.S. Pat. No. 6,872,050 to Nenstiel, assigned to the assignee of the present invention and incorporated herein by reference. A VGD is extended into the diffuser passage of the centrifugal compressor until a level of the measured signal drops below a predetermined threshold. The result is the elimination of stall and a corresponding drop in both internal and airborne sound levels. After a time, or a change in conditions, the diffuser gap is re-opened in steps or increments, until a level of the measured signal indicates that a stall condition is detected.
Stall and surge conditions are different physical phenomena that represent the extreme operating conditions of compressors. Stall is a local flow separation in one or more components of a compressor, and is characterized by discharge pressure disturbances at fundamental frequencies less than the rotational frequency of the impeller. Rotating stall in a centrifugal compressor is predominantly located in the diffuser and can be eliminated with the VGD. In contrast, surge is a system wide instability in a compressed gas system. The main flow in the compressor is momentarily reversed in direction and the surge is further characterized by even lower frequency, large pressure fluctuations.
A VGD includes a ring that can move to occupy a diffuser gap, which is an outlet flow path for compressed gas. The VGD can move from a retracted position, in which the ring is completely out of the outlet flow path to allow maximum gas flow, to an extended position, in which the ring occupies a portion of the outlet flow path, thereby restricting a portion of the gas flow. The ring is operable based on detection of conditions of stall in the centrifugal compressor. The variable geometry diffuser is used in conjunction with a probe or sensor that measures the alternating component of the discharge pressure to detect impending stall. The measured parameter is transmitted to a controller that is programmed to detect imminent stall based on this measured parameter. The controller then determines when it is necessary to activate the variable geometry diffuser in order to eliminate stall and as a consequence avoid surge. Thus, variable geometry diffusers have provided the advantage of stall avoidance, surge avoidance and reduction in noise that accompanies these conditions.
As part of a diffuser system within a centrifugal compressor, there are many options to recover static pressure downstream of the rotating impeller. The diffuser is responsible for reducing primarily the tangential component of refrigerant velocity, and secondarily, the radial component of refrigerant velocity. As the refrigerant velocity is reduced, the static pressure increases. A primary goal, where performance is critical, is to recover the static pressure with minimal total pressure losses.
Traditional diffusers in centrifugal compressors include vaneless, vaned (airfoil, wedge, high solidity or low solidity), pipe, tunnel and channel or a combination of these types. Each type of diffuser has its advantages and disadvantages. For example, vaneless diffusers consist of two walls, with no vanes on the high pressure side of the impeller. Static pressure recovery within a vaneless diffuser is a result of known relationships between entering velocity conditions and the radius ratio and width throughout the diffuser.
Various methods have been applied individually and in combination for capacity control, including PRV, also known as inlet guide vanes, pre-swirl vanes, etc. on the low pressure side of the impeller, variable speed drives, hot gas bypass, variable diffuser vanes and suction throttle valves. Each of these capacity control methods provides advantages and limitations. The most commonly utilized capacity control methods include PRV, hot gas bypass and variable speed drives.